Printed circuit boards and methods for manufacturing thereof for RF connectivity between electro-optic phase modulator and digital signal processor

ABSTRACT

A Printed Circuit Board (PCB) and methods for manufacturing the PCB board are provided. The PCB includes a Radio Frequency (RF) signal transition at a RF signal pad. Multiple conductive layers other than a conductive signal layer of the PCB and conductive portions of the conductive signal layer not in electrical contact with a RF signal transmission trace have common ground connections forming a ground cage structure within the PCB around the RF signal pad and RF the signal transmission trace.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This invention is a continuation of U.S. patent application Ser. No.16/890,057, filed Jun. 2, 2021, which is now U.S. Pat. No. 11,217,869with an issue date of Jan. 4, 2022, and which is also a continuation ofU.S. patent application Ser. No. 16/022,792, filed Jun. 29, 2018, whichis now U.S. Pat. No. 10,693,207, with an issue date of Jun. 23, 2021,the contents of each are incorporated by reference in their entirety.

TECHNICAL FIELD

This invention relates to high speed telecommunications, and inparticular to printed circuit board mounting of connectors operating atradio frequencies, components and systems operating at radio frequenciesand printed circuit board manufacture for operation at radiofrequencies.

BACKGROUND

Electrical signals modulated at Radio Frequencies (RF), for example ator below 50 GHz, are employed to provide high speed telecommunicationsat high data rates. Discontinuities in electrical conductors conveyingRF signals are prone to excite different signal resonance modes,especially in a millimeter waveband.

For example, within current electro-optical (E-O) interfaces betweensignal modulators and coherent Digital Signal Processing (DSP) chips,Gilbert's Push-On (GPO) and GPPO connectors are employed eitheredge-mounted or surface-mounted. FIGS. 1A, 1B and 1C show conventionalGPO and GPPO connectors mounted on a PCB board to convey signals betweenE-O Modulators and Coherent DSP chips. Conventional Printed CircuitBoard (PCB) connectivity attempts suffer from disadvantages and defectsincluding susceptibility to different signal resonance modes even in thelower frequency band of 10 GHz˜20 GHz.

FIG. 1A illustrates a GPPO connector surface-mounted to a PCB of asignal modulator card. GPPO connector 100, using a bent pin 102, issurface mounted 104 on a PCB pad 106. The bent pin 102 end PCBattachment is surrounded with rectangular array 108 of ground vias. Pin102 has two bends within connector body 100 for a total of three bendsincluding a bend for the pin 102 leading into a pad of the PCB trace.

FIG. 1B illustrates GPO connectors edge-mounted to a PCB of a signalmodulator component card. The PCB needs a rectangle cut-out to fit eachconnector body 110 on edge. GPO connectors 110 always employ an air gap114, about 0.5˜3 mils wide, between connector body 110 and the PCBcut-out necessary for component assembly.

FIG. 1C illustrates a conventional GPPO connector employing a similarair gap 114 between connector body and PCB cut-out.

The connector to PCB signal trace transition structures illustrated inFIGS. 1A, 1B and 1C have been found to make it very easy to excitedifferent RF signal resonance modes, especially in the millimeterwaveband. Such resonance results in large notches/ripples in insertionloss and return loss, and cause system failure due to the resultingfaulty transfer function. Such system performance failures incur hugecost due to product re-spin and deployment delays. This is a commonissue recognized in the art particularly in manufacturing coherent DSPline cards and E-O modules.

As an example, FIG. 2A illustrates lab measured resonance notches at 30GHz when using surface mounted GPPO connectors 100 as illustrated inFIG. 1A.

FIG. 2B illustrates factory measured resonance notches between 16 GHz 18GHz when using GPO connectors 110 edge-mounted to a PCB as illustratedin FIG. 1B.

FIG. 2C illustrates measured resonance notches between 16 GHz˜20 GHzwhen using an edge mounted GPPO connector with an air gap.

The above illustrated resonance notches in the transfer function couldnot be compensated out with current Finite Impulse Response (FIR) and/orpassive equalizers within a coherent DSP chip because they are toosharp. Such electrical interfaces between E-O modulator and DSP chipsare a “bandwidth bottleneck” for high speed telecommunications. There isa need to improve RF signal coupling into and out of a signal trace of aPCB via a RF connector.

SUMMARY

In general, transmission of RF signals can be provided by RF coaxialcables. A RF coaxial cable has an inner conductor surrounded by atubular insulating layer, which in turn is surrounded by a tubularconductive shield. An external tubular outer sheath or jacket providesphysical protection for the RF cable. RF cables are said to have atransmission line impedance, for example 50Ω.

RF connectors are electrical connectors intended to operate at radiofrequencies with reduced change in transmission line impedance. An RFconnector may connect a RF coaxial cable or another RF connector to anelectronic circuit, for example an electronic circuit on a PCB. The RFconnector maintains the RF shielding and transmission line impedancewithin the RF connector, however as described hereinabove conventionalconnection of a RF connector to PCB signal trace incurs signaltransmission discontinuities, for example signal transfer functions showresonance as illustrated in FIGS. 2A, 2B and 2C.

It has been found that such signal transmission discontinuities resultfrom discontinuities in the “inner conductor” between the RF connectorand the PCB signal trace, discontinuities in the “insulation” betweenthe RF connector and the PCB, and discontinuities in the “conductiveshielding” between the RF connector and PCB ground.

At least the above issues identified in the prior art, can be alleviatedby employing one of WSMP, G3PO and SMPS surface mount RF connector and aPCB signal pad structure to provide RF signal transition from the RFconnector to the PCB signal trace and vice versa. WSMP is a trademark ofRosenberger. GPO, GPPO and G3PO are trademarks of Corning Gilbert. SMPSis a trademark of Radiall. Generally the WSMP, G3PO and SMPS connectorsprovide a push-on connection without a threaded barrel.

In accordance with an aspect of the proposed solution there is provideda PCB having a plurality of dielectric layers distributed between aplurality of conductive layers. The PCB includes a RF signal transitionat a RF signal pad comprising: a RF signal transmission trace in aconductive signal layer other than a top and bottom conductive layers; ablind via providing electrical conductivity across at least onedielectric layer between the signal transmission trace and the signalpad; and a ground cage structure within the PCB around the RF signal padand the RF signal transmission trace, wherein the plurality ofconductive layers other than the conductive signal layer and conductiveportions of the conductive signal layer not in electrical contact withthe RF signal transmission trace have common ground connections.

In accordance with another aspect of the proposed solution there isprovided an electrical component including a Printed Circuit Board (PCB)having a Radio Frequency (RF) signal pad, the electrical componentcomprising: a RF connector having a signal pin oriented perpendicularlyto the RF signal pad on the PCB; and the PCB having a plurality ofdielectric layers distributed between a plurality of conductive layers,the PCB including a RF signal transition at the RF signal pad including:a RF signal transmission trace in a conductive signal layer other than atop and bottom conductive layers; a blind via providing electricalconductivity across at least one dielectric layer between the signaltransmission trace and the signal pad; and a ground cage structurewithin the PCB around the RF signal pad and the RF signal transmissiontrace, wherein the plurality of conductive layers other than theconductive signal layer and conductive portions of the conductive signallayer not in electrical contact with the RF signal transmission tracehave common ground connections, wherein the RF connector signal pin isconnected perpendicularly to the RF signal pad.

In accordance with a further aspect of the proposed solution there isprovided a PCB manufacture method comprising: forming a RF signaltransmission trace in a conductive signal layer of a first PCB core,wherein conductive portions of the conductive signal layer not inelectrical contact with the RF signal transmission trace spaced apartfrom the RF signal transmission trace to provide constant transmissionline impedance along the RF signal transmission trace; forming a firstanti-pad in the conductive signal layer around a terminal pad of the RFsignal transmission trace; forming a second anti-pad in a conductivelayer opposite the conductive signal layer of the first PCB core, thesecond anti-pad being concentric with the first anti-pad; depositing afirst laminate layer on top of the conductive signal layer; depositing atop conductive layer on top of the first laminate layer; forming a thirdanti-pad in the top conductive layer concentric with the first andsecond anti-pads; forming a RF signal pad in the top conductive layerconcentric with the terminal pad of the RF signal transmission trace;forming a blind via providing electrical connectivity between the RFsignal pad and the terminal pad of the RF signal transmission trace;forming a plurality of through vias in the plurality of conductivelayers other than the conductive signal layer and in conductive portionsof the conductive signal layer not in electrical contact with the RFsignal transmission trace; and plating the blind via and the pluralitythrough vias with conductive material, wherein a ground cage structurewithin the PCB is provided around the RF signal pad and the RF signaltransmission trace.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed solution will be better understood by way of the followingdetailed description of embodiments of the invention with reference tothe appended drawings, in which:

FIG. 1A is a schematic diagram showing a prior art GPPO connectorsurface mounted to a PCB;

FIG. 1B is another schematic diagram showing prior art GPO connectorsedge mounted to a PCB;

FIG. 1C is a schematic diagram showing another prior art GPPO connectoredge mounted to a PCB;

FIG. 2A is schematic graph showing lab measured resonance for thesurface mounted GPPO connectors as illustrated in FIG. 1A;

FIG. 2B is a schematic graph showing factory measured resonance for edgemounted DPO connectors as illustrated in FIG. 1B;

FIG. 2C is a schematic graph showing lab measured resonance for the edgemounted GPPO connector as illustrated in FIG. 1C;

FIG. 3A is a schematic diagram illustrating a plan cross-section view ofan RF connector and PCB signal pad structure in accordance with anembodiment of the proposed solution;

FIG. 3B is a schematic diagram illustrating an isometric cross-sectionview of the RF connector and PCB signal pad structure of FIG. 3A;

FIG. 4A is a schematic diagram illustrating an isometric partly cutoutview of the PCB signal pad structure of FIG. 3B in accordance with theembodiment of the proposed solution;

FIG. 4B is a schematic diagram illustrating an enlarged isometric partlycutout view of the PCB signal pad structure of FIG. 4A;

FIG. 4C is a schematic diagram illustrating an isometric see throughview of the PCB signal pad and PCB signal trace corresponding to thecutout views of FIGS. 3B, 4A and 4B;

FIG. 5A is a schematic diagram illustrating a top see through view ofthe PCB signal pad and PCB signal trace corresponding to FIG. 4B;

FIG. 5B is a schematic diagram illustrating a top plan view of the PCBsignal pad corresponding to FIG. 5A;

FIG. 6A is a schematic diagram illustrating an isometric view of PCB RFsignal transition structure providing four RF signal channels forsoldering a quad WSMP/G3PO/SMPS RF connector thereon;

FIG. 6B is a schematic diagram illustrating a top view of the PCB RFsignal transition structure of FIG. 6A;

FIG. 7 is a schematic diagram illustrating a PCB manufacturing method inaccordance with an implementation of the proposed solution;

FIG. 8 is a plot illustrating insertion loss measured results for animplementation in accordance with the proposed solution;

FIG. 9 is another plot illustrating return loss measured results for theimplementation of FIG. 8 ;

FIG. 10 is a further plot illustrating Time Domain Reflectometry (TDR)impedance measured results for the implementation of FIGS. 8 and 9 ; and

FIG. 11 is yet another plot illustrating Voltage Standing Wave Ratio(VSWR) measured results for the implementation of FIGS. 8, 9 and 10 ,

wherein similar features bear similar labels throughout the drawings.While the sequence described can be of significance, reference to “top”,“bottom”, “front” and “back” qualifiers in the present specification ismade solely with reference to the orientation of the drawings aspresented in the application and does not imply any absolute spatialorientation.

DETAILED DESCRIPTION

With the development of coherent technology, the data rate betweenCoherent DSP DAC outputs and E-O phase modulators is moving towards ahigher operating range between 56.8 Gb/s and 75 Gb/s in a singlechannel. The bandwidth of RF high speed electrical interconnects betweenCoherent DSP DACs and E-O phase modulators is an important factor whichinfluences overall optical system performance including transferfunction, chirp and Optical Signal-To-Noise Ratio (OSNR).

The proposed solution relates to RF signal transitioning from aWSMP/G3PO/SMPS RF connector to a Printed Circuit Board (PCB) E-O moduleand package where coherent DSP chips are located.

Such a surface mount WSMP, G3PO and SMPS RF connector 200 is illustratedin cross section in FIG. 3A, and an isometric view of the cross sectionis illustrated in FIG. 3B. While a two-part RF connector 200 isillustrated, the invention is not limited to two-part WSMP, G3PO andSMPS RF connectors.

With reference to both FIGS. 3A and 3B, in accordance with the proposedsolution, the number of bends in the RF signal path, bends whichcontribute to limit the total signal bandwidth, is reduced to two90-degree bends. Bend 202 is within the RF connector 200 from horizontalRF pin 204 to vertical pin 206. Another bend 208 is between RF connectorvertical pin 206 and the horizontal PCB transmission line 210 of the PCB212 at the signal pad 214.

For the first bend 202, connector manufacturing parameters are selectedfor the inner shape(s)/dimensions of the RF connector 200 to obtain nearideal 50 Ohms coaxial impedance at the bend 202. With the second bend208 in the signal path at connector-PCB transition zone, it is difficultto provide a structure having an inner conductor surrounded by a tubularinsulating layer at PCB 212.

In accordance with the proposed solution, a PCB multi-layerconfiguration is proposed to adjust and/or control the frequency of RFresonance modes out of an increased useful frequency band, to reduceparasitic parameters, and to decrease impedance discontinuity throughcurve-tuning line/stick shapes, spherical/cone-shaped transitionstructure and maintaining a coaxial-structure in transition.

In accordance with one embodiment, Table 1 provides a listing of PCBlayers (stack-up implementation) in the PCB 212. A person of ordinaryskill in the art would recognize that additional layers are notspecified such as antioxidation layers (Corrosion Inhibitor) coveringexposed copper top and bottom areas typically employed for long termuse. Specific details of PCB manufacture are omitted herein. It isunderstood that in accordance with another implementation the PCB stackup can include three Core layers and two Pre-Impregnated (Pre-Preg.)layers. Other implementations can include another number of copperlayers without departing from the proposed solution. For example,certain copper layers include Hyper Low Profile (HVLP) copper foil, VeryLow Profile (VLP) copper foil, Reverse-Treatment copper Foil (RTF). Itis understood that other laminates can be employed, such as but notlimited to Isola 370HR, instead of Pre-Preg. without departing from theproposed solution.

In accordance with the implementation listed in Table 1, the first rowin Table 1 specifies an ENIG (Electro-less Nickel Immersion Gold)/ImAg(Immersion Silver) plating employed to provide substantially resistancefree area for solder between the RF connector 200 to the PCB board 212to provide a solid ground return path connection. With respect toconducting layers of the PCB board 212, the first two rows of Table 1are regarded to specify a single conducting layer 1.88 mils thick. Forthe remainder of the description herein “L1” will be used to refer tothe combination of both top two rows in Table 1.

TABLE 1 Dielectric Cu Thick Thick Layer (mils) (mils) Layer TypeMaterial L1 1.5 Plating, with ENIG or ImAg L1 0.38 Foil (GND) Copper 4Pre-Impregnated Meg 4, Meg 6, Meg 7, Rogers3003, (Pre-Preg.) TU933, ParkMW4000, Tachyon100G L2 0.6 HVLP/VLP/RTF Copper (Signal/GND) 4 Core Meg4, Meg 6, Meg 7, Rogers3003, TU933, Park MW4000, Tachyon100G L3 0.6HVLP/VLP/RTF Copper (GND) 4 Pre-Preg. Meg 4, Meg 6, Meg 7, Rogers3003,TU933, Park MW4000, Tachyon100G L4 0.6 HVLP/VLP/RTF Copper (GND) 4 CoreMeg 4, Meg 6, Meg 7, Rogers3003, TU933, Park MW4000, Tachyon100G L5 0.6HVLP/VLP/RTF Copper (GND) 4 Pre-Preg. Meg 4, Meg 6, Meg 7, Rogers3003,TU933, Park MW4000, Tachyon100G L6 0.6 HVLP/VLP/RTF Copper (GND)

The PCB conductor layer stack-up is illustrated in FIGS. 3A and 3Bcollectively labeled PCB 212. It is understood that PCB 212 extendsfurther back from the RF connector 200 and extends further sideways(into and out of the page with respect to FIG. 3A) to provide electricconnectivity to other electrical components not shown such as but notlimited to a Coherent DSP of an E-O module. For ease of illustration ofthe proposed solution herein, the figures herein do not show the PCBdielectric layers listed in Table 1. However, the type, thickness,material composition and electromagnetic properties of the dielectriclayers are important in providing insulation properties between thesignal path, and a ground cage around the RF signal path between the pin206 of the RF connector and the PCB transmission line 210 to providehigh speed/low loss operation. Megtron 4, Megtron 6 and Megtron 7 aretrademarks of Panasonic. RO3003 (Rogers3003) is a trademark of RogersCorporation. TU-933 is a trademark of Taiwan Union TechnologyCorporation. Meteorwave 4000 (Park MW4000) is a trademark of ParkElectrochemical. Tachyon 100G is a trademark of Isola Group.

PCB Signal Transmission Trace

In accordance with the example implementation illustrated throughout thefigures, L2 has a signal layer type (Signal/GND). Without limiting theinvention, a PCB signal transmission trace is lithographicallymanufactured in the copper layer L2 to route an RF signal along a signalpath to/from other components (not shown) on the PCB board. In otherimplementations, the PCB signal transmission trace can be manufacturedin a different copper layer other than the top and bottom copper layersof the PCB 212. At least one upper and lower copper layer with respectto the signal path is used to provide RF shielding below and above alongthe PCB signal transmission trace 210. In accordance with theillustrated implementation, as best illustrated see-through in FIG. 4B,PCB signal transmission trace 210 strip line is routed out at PCB layerL2 with ground reference planes at layers L1 and L3. Within the samelayer L2, 42 mils ground clearance is provided on both sides of the PCBsignal transmission trace strip line 210. RF shielding is provided tothe sides along the PCB signal transmission trace 210 by groundedportions 220 of the L2 copper layer. High density ground stitching isprovided by vias 222 at least through layers L1, L2 and L3 along the PCBsignal transmission trace 210. In the figures, without limiting theinvention, ground stitching vias 222 are through-vias shown drilledthrough all layers L1 to L6 of PCB 212, for example 10 mils drill (20mils diameter pad) and 30 to 75 mils apart. In other implementationsblind vias can be employed.

In the transition at layer L2, the PCB signal transmission trace 210 isconfigured to have a tuned tapered shape 224 expanding to a 22 milsterminal pad to provide an impedance matched transition at highfrequency (detail in FIGS. 4B and 5A) to a signal pad 214. While in thefigures the dielectric layers are omitted to provide see-throughillustration of the proposed solution, the tuned taper 224 lies on adielectric layer (i.e. not floating).

PCB Signal Pad

SMT pad 214 is provided at layer L1 for center signal vertical conductorpin 206 of the RF connector 200 to be soldered thereto. For example, theconnector pin 206 is soldered during oven re-flow to SMT pad 214 on thePCB 212. For example, the SMT pad 214 has a 16 mils diameter at PCB toplayer L1. This transition transfers the signal path to PCB signaltransmission trace strip line 210 on PCB layer L2.

During PCB layer manufacturing, a blind-via 216, best illustrated inFIGS. 4A and 4B, is laser drilled in the top copper layer and topdielectric layer to the terminal pad of the PCB signal transmissiontrace 210. For example, the drilled via diameter dimension is 6 mils.After plating the blind-via 216, the blind-via 216 can be filled withconductive paste such as CB-100 or non-conductive epoxy ink. The SMT pad214 provided during L1 finishing (ENIG/ImAg) is plated with zero stub.While the dielectric layers are omitted to provide see-throughillustration of the proposed solution, the 16 mills diameter SMT pad 214rests on the top of a dielectric layer (i.e. not floating).

PCB Ground Cage Structure

In accordance with the proposed solution, the PCB copper layers at asignal pad on a PCB are contoured during PCB manufacture, for examplethrough PCB lithography, to provide a ground cage around the PCB signalpad and PCB signal transmission trace. With reference to Table 1, layersL1 through L6 have a ground layer type (GND) away from and around thePCB signal transmission trace 210.

Around the SMT pad 214 (and blind via 216) the multiple ground layers ofthe PCB 212 are contoured in the plane of each corresponding copperlayer with selected “anti-pad” diameters for different ground layers.With reference to the inset to FIG. 4B collectively the inner edges ofthe contoured copper layers can form a spherical-shaped ground structureproviding impedance matching in the transition. Without limiting theinvention, such a spherical-shaped ground cage structure can be providedby circular ground layer contouring, for example for the six layerslisted in Table 1 having: 51 mils anti-pad diameter 232 at layer L1, 51mils anti-pad diameter 234 at layer L2, 47 mils anti-pad diameter 236 atlayer L3, 43 mils anti-pad diameter 238 at layer L4, 24 mils anti-paddiameter 240 at layer L5, and a solid ground plane at layer L6. Inaccordance with another implementation, the ground cage structure withinthe PCB around the SMT pad 214 is conical.

Ground through vias 230 are drilled around the SMT pad 214 (FIGS. 4A, 4Band 4C). For example, seven vias 230 of 10 mils diameter drill (20 milsdiameter pad) are located evenly around the central pad 214 at a 36 milsradius. This provides a PCB ground cage having a coaxial shape of havingabout 50 Ohms impedance for cylindrical signal propagation.

Impedance Matching and Signal Discontinuity Control

It has been discovered that an impedance discontinuity from thetransition of the RF signal at PCB ground cage close to SMT pad 214 intothe PCT signal transmission trace strip line 210 can be compensated byPCB ground layer contouring.

In accordance with the proposed solution, ground reference planes areextended at layers neighboring the PCB transmission trace strip line210. In accordance with the illustrated implementation, ground referenceplanes at layers L1 and L3 are extended into the volume of the PCBground cage structure. For example, FIGS. 4A, 4B, 5A and 5B illustrateground plane extension along the direction of the PCB transmission tracestrip line 210 as perpendicular edge 246 10 mils away from SMT pad 214at layer L3 and perpendicular edge 248 20 mils away from SMT pad 214 atlayer L1. Other layers can be similarly extended to tune impedance inthe transition.

FIGS. 6A and 6B illustrate the proposed solution employed to provide aRF signal transition from a quad WSMP/G3PO/SMPS connector (not shown) tofour PCB signal transmission traces of a coherent E-O module. Theillustrated extent of the top layer L1 corresponds to the ENIG or ImAgplated area under the WSMP/G3PO/SMPS connector. It is understood thatthe PCB 212 extends further to the sides and towards the back. Throughholes 250 are employed to position the WSMP/G3PO/SMPS connector toregister vertical pins 206 with SMT pads 214.

Method of PCB Manufacture

With reference to FIG. 7 and Table 1, in accordance with a preferredembodiment of the proposed solution, a method of PCB manufacture 300includes the following steps some of which are understood by a person ofskill in the art to be performed in parallel. Methods of PCB manufactureare understood to relate to PCB fabrication. While the methods of PCBmanufacture are described herein with reference to depositing resist, itis understood that the methods can alternatively include laser directimaging techniques.

A PCB core having copper layers L2 and L3 is provided 302. Lithographictechniques are employed to deposit 304 a resist over layer L2 exposingthe anti-pad 234 away from the taper 224 and exposing the groundclearance 220 along the PCB signal transmission trace 210. Resist isalso deposited 306 over layer L3 exposing the copper between anti-pad236 and ground plane extension to edge 246. Exposed copper in layers L2and L3 is etched 308 away.

A PCB core having copper layers L4 and L5 is provided 312. Lithographictechniques are employed to deposit 314 a resist over layer L4 exposingthe anti-pad 238. Resist is also deposited 316 over layer L5 exposingthe anti-pad 240. Exposed copper in layers L4 and L5 is etched 318 away.

The two PCB cores are laminated 320 using Pre-Preg. between layers L3and L4. Pre-Preg. is deposited 322 on layers L2 and L5. Copper layer L6is deposited 324.

Copper layer L1 is deposited 330. Resist is deposited 332 over layer L1exposing the copper between pad 214, anti-pad 232 and ground planeextension to edge 248. Exposed copper in layer L1 is etched 334 away.Blind-via 216 is laser drilled 336 exposing L2. Blind-via 216 is platedand filled 338 with one of conductive paste such as CB-100 with zerostub (planarized). Alternatively, the blind-via 216 can be filled withnon-conductive epoxy ink.

Ground stitching vias 222 and 230 are drilled 340 and plated/filled 342.The ground stitching vias 222 and 230 can be filled with one ofconductive paste and non-conductive epoxy ink. Layer L1 is selectivelyplated 344 with ENIG or ImAg. Solder paste is deposited 346 over theENIG/ImAg exposed area and SMT pads 214. Positioning holes 250 aredrilled 348.

An WSMP/G3PO/SMPS connector 200 is positioned 350 on top with verticalpins 206 registered over SMT pads 214. The PCB 212 and RF connector 200are placed in an oven for solder re-flow 252.

The preferred PCB manufacture method has been found improve productionyield.

In accordance with another method, the above PCB manufacture steps canbe re-sequenced to employ three PCB cores laminated with two layers ofPre-Preg.

Characterization Measurements

The combination of elements and techniques of the proposed solution hasbeen tested. Based on measurements, the transition design isresonance-free up to 60 GHz as illustrated in FIG. 8 . FIG. 9illustrates return loss better than −20 dB up to 57 GHz. FIG. 10illustrates a measured Time Domain Reflectometry (TDR) impedance between49.7 Ohms˜50.1 Ohms. TDR measures reflections that result from a signaltravelling through the PCB RF signal transmission trace 210 andconnector 200. FIG. 11 illustrates measured Voltage Standing Wave Ratio(VSWR) less than 1.18 up to 53 GHz. VSWR is a measure of the efficiencyof radio-frequency power transmission from a power source through atransmission line into a load (for example, from a power amplifierthrough a transmission line). These measurements have been found toagree with simulation results.

The proposed solution provides good broadband of operation for improveddata transmission of Non-Return-to-Zero (NRZ)/Return-to-Zero(RZ)/Four-level Pulse Amplitude Modulation (PAM4) signals at rates up to100 Gbps (50 GHz for first Nyquist frequency spectrum). When theproposed solution is used in an optical coherent solution, improvedoptical performance is provided with transmitter path flatness withoutnotch up to 60 GHz, <20 dB return loss up to 57 GHz.

While the invention has been illustrated and described with reference topreferred embodiments thereof, it will be recognized by those skilled inthe art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A Printed Circuit Board (PCB) comprising: aplurality of layers; a signal pad, at one of the plurality of layers,connected to a signal transmission trace, wherein the signal pad isdirectly connected to a surface mount Radio Frequency (RF) connectorthat interfaces an RF signal with the signal pad; and a plurality ofvias surrounding the signal pad such that the plurality of vias forms aPCB ground cage structure disposed through the plurality of layers,wherein the PCB ground cage structure through the plurality of layersincludes each of the plurality of layers having a contour around thesignal pad with at least two of the plurality of layers having differentdiameters for their associated contours.
 2. The PCB of claim 1, whereinthe PCB ground cage structure includes a coaxial shape through theplurality of layers.
 3. The PCB of claim 1, wherein the PCB ground cagestructure includes a ground reference plane at one or more of theplurality of layers.
 4. The PCB of claim 3, wherein the ground referenceplane is extended, into the PCB ground cage structure, at layersneighboring the signal transmission trace.
 5. The PCB of claim 1,wherein the Radio Frequency (RF) signal is up to 50 GHz.
 6. The PCB ofclaim 1, wherein a signal path between the surface mount RF connector tothe signal pad has two 90-degree bends.
 7. The PCB of claim 1, whereinthe plurality of layers about the signal pad are shaped in a transitionstructure for impedance matching.
 8. The PCB of claim 1, wherein thesurface mount RF connector is one of a WSMP, G3PO and SMPS surface mountRF connector.
 9. The PCB of claim 1, wherein the signal transmissiontrace includes a tapered shape as it connects to the signal pad.
 10. ThePCB of claim 1, wherein the signal transmission trace is soldered to thesignal pad.
 11. A Printed Circuit Board (PCB) comprising: a plurality oflayers; a signal pad, at one of the plurality of layers, connected to asignal transmission trace, wherein the signal pad is directly connectedto a surface mount Radio Frequency (RF) connector that interfaces an RFsignal with the signal pad; a ground reference plane at one or more ofthe plurality of layers; a plurality of vias surrounding the signal padand each positioned around the signal pad such that the plurality ofvias forms a PCB ground cage structure disposed through the plurality oflayers, wherein the PCB ground cage structure through the plurality oflayers includes each of the plurality of layers having a contour aroundthe signal pad with at least two of the plurality of layers havingdifferent diameters for their associated contours.
 12. The PCB of claim11, wherein the PCB ground cage structure includes a coaxial shapethrough the plurality of layers.
 13. The PCB of claim 11, wherein theground reference plane is extended, into the PCB ground cage structure,at layers neighboring the signal transmission trace.
 14. The PCB ofclaim 11, wherein the Radio Frequency (RF) signal is up to 50 GHz. 15.The PCB of claim 11, wherein a signal path between the surface mount RFconnector to the signal pad has two 90-degree bends.
 16. The PCB ofclaim 11, wherein the plurality of layers about the signal pad areshaped in a transition structure for impedance matching.
 17. The PCB ofclaim 11, wherein the surface mount RF connector is one of a WSMP, G3POand SMPS surface mount RF connector.
 18. The PCB of claim 11, whereinthe signal transmission trace includes a tapered shape as it connects tothe signal pad.
 19. The PCB of claim 11, wherein the signal transmissiontrace is soldered to the signal pad.